The present invention relates generally to high definition television (HDTV) systems and particularly concerns an HDTV system in which a broadcast video signal can be reproduced by different receivers at different levels of resolution.
Numerous HDTV systems have been proposed for transmitting a wideband video signal through a channel of limited bandwidth, such as a standard 6 MHz television channel. Since the HDTV source video signal typically has a bandwidth on the order of 37 MHz, the signal must be substantially compressed prior to transmission. Various data compression techniques have been proposed for this purpose including a number of techniques based on sub-band coding. Sub-band coding, as the term is used herein, refers generally to a process wherein the source video signal is converted into a plurality of spectral coefficients either by spatially filtering the video signal or by subjecting it to a suitable block transform, such as the discrete cosine transform (DCT). In either case, the video signal is separated into a plurality of sub-bands each comprising a series of spectral coefficients, with the coefficients derived for each sub-band representing a different spectral component of a respective block of the video image. The sub-band coefficients are subsequently processed in accordance with a data compression algorithm to allow for their transmission within the available bandwidth of the transmission channel. In order to reproduce the transmitted signal, a block of coefficients is derived for each block of the video image and the coefficient blocks are processed in an inverse sub-band coder to reconstruct a representation of the source video signal.
Normally, an identical block structure is used both at the transmitter to encode the video signal and at the receiver to decode it. For example, the source video signal may be encoded using an (8 .times.8) block structure wherein a block of 64 spectral coefficient terms are generated corresponding to each (8.times.8) pixel block of the source video signal. Each coefficient term is generated in a process which is the equivalent of multiplying each pixel in a given pixel block by a respective value of a selected 64-value basis function within a given sampling interval, and then summing the results. At the receiver, the reconstructed image is obtained by decoding successive blocks of 64 spectral coefficient terms to derive corresponding (8.times.8) pixel blocks for display. In the receiver, each pixel is derived in a process which is the equivalent of multiplying each coefficient by 64 corresponding values of 64 different inverse basis functions within the given sampling interval, and then summing the results. In this example, processing circuits operating at speeds of about 80 MHz in both the transmitter and receiver ar required. Processing circuits capable of operating at such high speeds are quite costly and therefore pose a limitation on the near-term introduction of reasonably priced HDTV receivers. Display devices, e.g. CRT's, capable of displaying an image with the resolution afforded by the HDTV signal are also quite costly and therefore impose another limitation on the introduction of reasonably priced HDTV receivers.